DEVELOPMENT OF BONE. 77 



above described. New bone thus formed at the sur- 

 face appears at first by no means in the form of 

 smooth, continuous layers, for as the blood-vessels 

 and connective-tissue bundles, along which the oste- 

 oblasts lie, are arranged at varying angles with the 

 surface of the bone and with each other, the effect 

 is to produce irregular-branching cavities, upon 

 whose walls the new layers of bone are deposited. 

 When these branching cavities become filled, with 

 the exception of the space occupied by the blood- 

 vessels and marrow-tissue, by successive lamellae of 

 bone, they constitute the structure w r ith which we 

 are already familiar under the name of Haversian 

 canals and Haversian lamellae. Where the forma- 

 tion of bone has taken place along the bundles of 

 connective-tissue, these bundles sometimes persist 

 for a long time, in a modified form, among the 

 lamellae, and constitute the above-mentioned SAar- 

 pey s fibres. 



Thus, by the transformation of cartilage and ap- 

 position at the surface, the long bones are formed. 

 In these bones the ossification progresses toward the 

 epiphyses, where independent centres of ossification 

 are established. The lines of ossification approach 

 each other, and finally, when the process of growth 

 in the bone is complete, the band of cartilage which 

 separated them disappears, and epiphysis and dia- 

 physis join to form a single bone. As the bone 

 grows by apposition beneath the periosteum, the 



